Topic 2: Bonding and Structure

Question 1

what is ionic bonding?

Answer 1

ionic bonding is the strong, net electrostatic attraction between oppositely charged ions

Question 2

What kind of structure do ionic compound such as NaCl have?

Answer 2

Giant, regular ionic structure, with a cubic shape and giant repeat pattern

Question 3

Do ionic compounds dissolve in water?

Answer 3

Most dissolve in water as water is polar. The slightly + H is attracted to anions and the slightly - O attracts cations, breaking the structure. The water molecules surround the ions in a process called hydration.

Question 4

Explain reasons for the trends in ionic radii down a group

Answer 4

Ionic radii increases down the group because of more electron shells. This means there is more electron shielding.

Question 5

Explain reasons for the trends in ionic radii for a set of isoelectric ions

Answer 5

For isoelectric ions, as atomic number increases, ionic radius decreases.
This is bc the attractive force from the nucleus increases due to more protons, so it pulls the other shell more in.

Question 6

What is some evidence for the existence of ions?

Answer 6

The physical properties of ionic compounds provide evidence for ions. Eg molten ionic compounds conduct electricity and undergo electrolysis such as sodium chloride.
+ Na ions are reduced at the cathode
- Cl ions are oxidised at the anode

Question 7

Why are metal ions smaller than metal atoms?

Answer 7

Metal ions are smaller than metal atoms due to the loss of the outer electron shell and increased proton:electron ratio. Therefore there is a stronger attraction between the nucleus and each electron and ionic radius decreases.
The opposite is true for non metals

Question 8

State the definition for a covalent bond

Answer 8

cv bonds are formed by the overlap of 2 atomic orbitals that each contain a single electron. It’s a strong electrostatic attraction.
Shared pair of e-

Question 9

What are single, double and triple covalent bonds?

Answer 9

single= 1 shared e pair
double=2 shared e pairs
triple=3 shared e pairs

Question 10

What is a dative covalent bond?

Answer 10

1 atom donates 2 of its electrons to an atom or ion to form a bond. Eg H+ ion bonds w/ NH3 to form NH4. NH3 gives 2 e to share w/ H+. An arrow represents the dative/coordinate bond

Question 11

What kind of structure does CO have?

Answer 11

It has a double covalent bond AND a dative covalent bond. so in the overlap of the dot cross diagram draw 2 pairs AND 2 dots. These 2 dots came from the oxygen.

Question 12

What is bond strength? How do you increase it?

Answer 12

Bond strength is the amount of energy required to break one mole of bond in the gaseous state.
For similar bonds, shorter bond = greater strength. This is bc closer bonds mean more electrostatic attraction between the two nuclei and the electrons in overlapping orbitals.
Lone pairs make bonding weaker as they repel each other

Question 13

What is the bond length? Explain how bond length arises

Answer 13

Bond length: distance entre nuclei of 2 atoms covalently bonded.
In covalent molecules there are attractive forces between nuclei and the shared e. Also, there are repulsive forces between the 2 nuclei and between e. Theres a balance between these forces, resulting in bond length

Question 14

How does a higher e density increase bond strength?

Answer 14

A higher e density increases the attractive force between atoms.
Therefore atoms are pulled in further towards each other.
This leads to shorter bond length, increasing bond strength and therefore increasing bond enthalpy

Question 15

How do you determine the shape of a simple molecule or ion?

Answer 15

Draw the dot cross diagram to find the no of lone pairs and bond pairs.
For cations, remove e from the central atom eg: in NH4+ N would have 4 e instead of 5 as it is a + ion
For anions, add e to the central atom.
If the dot cross has a lone pair, subtract 2.5 from the bond angle. If hay 2 lone pairs, subtract 2 x 2.5= 5
ALWAYS draw in 3D! (dashed lines etc)

Question 16

Why do bonds repel each other? Explain why molecules have a certain shape and bond angle.

Answer 16

Bonds have electrons so they want to be as far apart as possible. Bond pairs repel each other equally.
If a molecule has lone pairs, these repel more than bond pairs.
Molecules have a certain shape/bond angle to achieve minimum repulsion between pairs of electrons.

Question 17

Describe a tetrahedral shape

Answer 17

CH4 is tetrahedral, with a bond angle of 109.5. Hay 4 bonding pairs and zero lone pairs.

Question 18

How many bond pairs and lone pairs do linear shapes have?

Answer 18

2 bond pairs, no lone pairs. Bond angle is 180°

Question 19

Describe a trigonal pyramidal shape

Answer 19

NH4 is trigonal pyramidal, Bond angle 107.

3 bond pairs and 1 lone pair.

Question 20

Describe a Trigonal planar shape

Answer 20

SO3 is a trigonal planar. Three bonding pairs, no lone pairs. The bond angle is 120°

Question 21

Describes a trigonal bipyramidial shape

Answer 21

PCl5 Is the trigonal bipyramidal. Five bonding pairs, no lone pairs. The bond angles are 90° and 120°

Question 22

Describe a bent shape

Answer 22

Two bond pairs and two lone pairs. Example of this is water. Bond angle is 104.5°

Question 23

What must occur for a substance to dissolve?

Answer 23

The solvent and substance bonds must break.
New bonds are formed between the solvent and the substance.
The new bonds formed must be the same strength or greater than those broken, otherwise the substance won’t dissolve.
Eg Al2O3 has ionic bonding too strong to be broken so its insoluble despite being ionic.

Question 24

What are polar solvents?

Answer 24

They have polarity.
Some have H bonds like water, called an aqueous solvent.
Some have dipole dipole and London forces, called nonaqueous solvents

Question 25

Can alcohols dissolve in polar solvents?

Answer 25

Alcohols dissolve in polar solvents as they can hydrogen bond with water molecules. The hydrocarbon part is nonpolar so it won’t dissolve. The bigger the hydrocarbon, the less soluble the alcohol is

Question 26

Why can’t haloalkanes dissolve in water?

Answer 26

Haloalkanes don’t dissolve in water because their dipoles aren’t very strong.
Water forms strong hydrogen bonds between each other more than with haloalkanes so they’re insoluble.
Haloalkanes can dissolve in substances with permanent dipole dipole interactions

Question 27

What are nonpolar solvents?

Answer 27

No polarity. Only have London forces.

Alkanes dissolve best in nonpolar solvents as they form Ldn forces between molecules.

Question 28

What is electronegativity?

Answer 28

electronegativity is the tendency of an atom or functional group to attract a bonding pair of electrons.
Generally electronegativity decreases down a group because of shielding, and increases across a period.

Question 29

explain hydrogen bonds

Answer 29

H bonding occurs when a H atom in a very polar CV bond is attracted to a very small, highly electronegative atom, either N, O or F.
They’re stronger than London forces, increasing bpt.
Show H bonds using using dotted lines between lone pairs and H.
Molecules w H bonding also have Ldn and permanent dipole dipole forces.

Question 30

How do polar bonds form? Are there cases where some polar molecules might not be polar?

Answer 30

CV bonds can be polar if hay a difference in electronegativity. The greater the difference, the more polar the bond. Polar means an uneven distribution of charge.
BUT if polar bonds are arranged symmetrically then you have no overall polarity eg in C02.

Question 31

What type of force is in between simple covalent substances?

Answer 31

simple covalent substances arent entirley isolated- hay 3 types of intermolecular forces between them: London, permanent dipole-dipole forces and H bonds

Question 32

Describe London forces and how they arise

Answer 32

AKA VDW forces/ instantaneous forces/ induced dipole-dipole interactions. Only occur entre mollecular substances and noble gases, NOT between ionic substances.
London forces are so weak that even 0 degrees is enough to overcome them.

Temporary asymmetrical distribution of charge creates an instantaneous/temporary dipole. These produce induced dipoles which form London forces.

Question 33

Explain evidence for London forces

Answer 33

When a noble gas is cooled, they become liqid and then solid.
This would be impossible si no era attractions between atoms. Therefore these must be London forces

Question 34

Describe a temporary dipole or induced dipole

Answer 34

Usually the e cloud is spread evenly round the nucleus. But, occasionally the e cloud fluctuates so its not evenly spread round the nucleus.
This fluctuation means the atom now has one slightly - and slightly + side, called a dipole.
Because it is only fleeting, it is called a temporary dipole.
These fluctuations in the e cloud can cause dipoles in other atoms, called induced dipoles.

Question 35

Why are contour lines closer to chlorine in a molecule of HCL?

Answer 35

Contour lines are closer to chlorine because it has greater electronegativity. Because of this the charge is slightly negative. This means that the hydrogen is slightly positive, creating a polar covalent bond as bonding electrons are unequally distributed.

Question 36

explain permanent dipole-dipole attractions

Answer 36

Polar molecules have electrostatic attractions between opp charges on neighbouring molecules, eg HCL gas: hay permanent dipole-dipole forces between slightly + H and slightly - Cl of ANOTHER HCl molecule.
Unlike ldn forces dipole dipole interactions involve molecules w a permanent dipole so are stronger.
The molecules also have ldn forces, affecting bpts, solubility and viscosity.

Question 37

What type of bonds do S and P orbitals form?

Answer 37

S orbitals form single sigma bonds. P orbitals form double pi bonds. Pi bonds cannot form until the sigma bond has formed

Question 38

Explain covalent bonding in H2

Answer 38

S shells overlap to form a sigma bond.

The two electrons exist in the new orbital and the highest electron density is found between the two nuclei

Question 39

Explain covalent bonding in the chlorine molecules

Answer 39

1 P shell overlaps to form a Sigma bond. 2p shells must overlap to form a pi bond

Question 40

Explain the covalent bonding in Ethene and use it to explain why alkenes are more reactive than alkanes

Answer 40

A pi bond is formed from the sideways overlap of p orbitals.
This results in high electron density both above and below the molecule. The pi bond is weaker than the sigma bond. This is why alkenes are more reactive than alkanes

Question 41

Describe and explain the trend in boiling temperature of alkanes as the chain length increases

Answer 41

As chain length increases there are more electrons so higher amounts of london forces.
There are also more points of contact in between molecules . Greater overall london forces, means more energy needed to break the intermolecular forces of attraction

Question 42

Why would alkanes with branched chains have lower boiling points than those with straight chains?

Answer 42

Branched chains are less easily slotted together, so fewer points of contact between molecules. Therefore less london forces, less energy needed to break the intermolecular forces of attraction

Question 43

Why do alcohols like ethanol have higher boiling temperatures than alkanes with the same numbers of carbons and similar numbers of electrons

Answer 43

OH group in alcohols mean H bonds form. Much stronger than ldn forces so more energy needed to break the intermolecular forces of attraction

Question 44

Describe and explain the trends in boiling temperatures of the hydrogen halides from HF to HI

Answer 44

HF has hydrogen bonds as F is so electronegative, and there is more electron density with fluorine.
From Cl to I there are more electrons and therefore more ldn forces, so after massive dip, bpts slowly increase again.

Question 45

Why do bpts increase down group 7? Reference dipoles in your answer

Answer 45

Down the group, molecules get bigger, with more e. More e means more Ldn forces that need more energy to overcome.
As atomic radius increases down the group, outer e become further from the nucleus, and less strongly attracted to it. Therefore, temporary dipoles are easier to induce.

Question 46

State the definition for discrete molecules

Answer 46

An electrically neutral group of two or more atoms held together by chemical bonds

Question 47

Why is the melting point of magnesium oxide greater than potassium chloride?

Answer 47

Both Mg and O have smaller ionic radii than K and Cl. Smaller ions= stronger bond.
Also Mg and O have a 2+ and 2- charge, which is higher than K+ and Cl-. Higher charge on ions= stronger bond.
Stronger bonds need more energy to break, so MgO has a higher mpt

Question 48

Explain polarisation from ionic bonds.

Answer 48

+ Ions attract the electron in the negative ion, polarising it. The ions affect those next to them.

Question 49

What makes an ion more polarising?

Answer 49

Small and positive ions are polarising. Large and negative ions are polarised. Ionic radius is more important to consider. The smallest cation and largest anion together in a compound would show the most covalent characteristics

Question 50

what is metallic bonding?

Answer 50

metallic bonding is the strong electrostatic attraction between metal ions and the delocalised electrons. It has a giant metallic structure

Question 51

Why is ice less dense than water?

Answer 51

Ice forms a regular structure held by H bonding. Molecules are further apart so ice is less dense.

Question 52

Why can’t non polar substances dissolve in water?

Answer 52

Non polar substances don’t dissolve in water as water forms stronger H bonds between each other than interacting with nonpolar molecules.

Question 53

Explain why the bond angle around the hydrogen atom involved in a hydrogen bond is 180°

Answer 53

There are 2 bonds around the H atom. These repel to achieve minimum repulsion. This forms a linear shape with a bond angle of 180.